Exhaust gases from diesel engines contain fine particles (particulate matter) mostly comprising carbon (soot, etc.) and high-boiling-point hydrocarbons, which are likely to adversely affect humans and the environment when discharged to the air. Accordingly, a ceramic honeycomb filter (hereinafter referred to as “honeycomb filter”) for removing the particulate matter to clean the exhaust gas has conventionally been mounted in an exhaust pipe connected to a diesel engine. As shown in FIG. 2, a honeycomb filter 20 comprises a ceramic honeycomb structure 21 comprising porous cell walls 7 forming large numbers of flow paths 5, 6 and a peripheral wall 8, and plugs 11a, 11b alternately sealing both ends 9, 10 of the flow paths 5, 6 in a checkerboard pattern.
A method widely used for the production of the above honeycomb structure 21 comprises the steps of blending a mixture of a ceramic material, a molding aid, a pore-forming material for forming pores in the cell walls, etc. with water to prepare a moldable material for forming a honeycomb extrudate (hereinafter referred to simply as “moldable material”), and extrusion-molding and sintering the resultant blend to form a honeycomb structure. In the honeycomb structure used as a filter, the pore diameter and the porosity should be controlled to improve the efficiency of capturing particulate matter in the exhaust gas and reduce pressure loss. The control of the pore diameter and the porosity has conventionally been conducted by adjusting the amount of a pore-forming material such as coal powder, wheat powder, etc. To obtain honeycomb structures having smaller thermal expansion and higher porosity, microcapsules (gas-containing resin balloons) described, for instance, in JP 2003-38919 A have recently got used as a pore-forming material.
In the step of blending a mixture of a ceramic material, a pore-forming material, etc. with water, it is important to uniformly blend the moldable material while removing as much air as possible from the moldable material. If the moldable material were not uniformly blended, or if air were not sufficiently removed from the moldable material, the resultant honeycomb structure would have defects.
As a technology for uniformly blending a moldable material to reduce the amount of air remaining in the moldable material, JP 2005-271395 A discloses a screw-type, vacuum-blending machine comprising a first drum comprising a divider having pluralities of flow paths for finely dividing the moldable material at an outlet, and a second drum in which the moldable material coming from pluralities of flow paths of the divider is degassed in vacuum while blending. However, because microcapsules (gas-containing resin balloons) are less resistant to a shearing force, they are likely broken by an excessive shearing force exerted during blending when the screw-type blending machine described in JP 2005-271395 A is used, resulting in a honeycomb structure with small porosity. When microcapsule shells are as thin as 0.1-2 μm, the microcapsules are easily broken. This is remarkable particularly when the shells are as thin as 0.1-0.8 μm, resulting in a honeycomb structure with extremely small porosity. When a large amount of microcapsules are added to the moldable material to obtain the desired porosity, the moldable material has a small viscosity, resulting in an extrusion-molded honeycomb with poor shape retention, and thus poor dimensional accuracy. It is thus difficult to produce, for instance, a large honeycomb structure having a diameter of 180 mm or more in a cross section perpendicular to flow paths, and large porosity. Although blending with a low speed and a low shearing force reduces the breakage of microcapsules, providing a honeycomb structure with large porosity, a uniform moldable material cannot be obtained because of insufficient blending. Such non-uniform moldable material contains small portions with different fluidity, providing an extrusion-molded honeycomb with breakage and/or undulation in cell walls. A ceramic honeycomb filter formed from such a non-uniformly molded body suffers a low particulate-matter-capturing ratio due to the breakage of cell walls, and large pressure loss due to the undulation of cell walls.
JP 2006-264237 A discloses a method for producing molded honeycomb by blending starting materials containing ceramic materials in a blending machine to form a moldable material, which is extruded and cut to a desired length, and then extruded from a piston-type extruder. Even with the blending machine described in JP 2006-264237 A, microcapsules are broken by an excessive shearing force exerted during blending, resulting in a honeycomb structure with small porosity.
In a method using a kneader comprising two rotors that rotate for blending, air is likely introduced into the resultant moldable material, providing a honeycomb structure with defects. Accordingly, the blending of moldable materials with a kneader is not common.